Nuclear magnetic resonance (NMR) technologies, such as NMR spectrometers and imaging systems, allow researchers to observe certain magnetic properties of atomic nuclei. These observations can be used to study basic chemical and physical properties of molecules or other small objects. NMR technologies are commonly used, for instance, to perform research on organic and inorganic molecules in the fields of medicine, chemistry, biology, and pharmacology.
NMR measurements are typically performed by an NMR probe that receives a sample to be studied. The sample is placed in a static magnetic field which aligns the magnetic dipoles of its atomic nuclei. Thereafter, the NMR probe applies a time-varying radio-frequency (RF) magnetic field to the sample to perturb the alignment of the magnetic dipoles. Next, the NMR probe detects the magnetic field generated by the perturbed nuclei as they return to their aligned positions. Finally, the detected magnetic field is analyzed to identify various aspects of the sample, such as its composition, the structure of its molecules, and other valuable information.
The NMR probe typically comprises a probe coil that generates the time-varying magnetic field to be applied to the sample and/or detects the magnetic field generated by the perturbed atomic nuclei as they return to their aligned positions. These magnetic fields typically oscillate in the radio-frequency (RF) range. Accordingly, the probe coil may be referred to as an RF transmitter coil, an RF receiver coil, or an RF transmitter/receiver coil.
To properly perturb the atomic nuclei, the probe coil should generate the time-varying magnetic field at the resonance frequency of the atomic nuclei. In addition, to accurately detect the magnetic field generated by the atomic nuclei, the probe coil should be tuned to detect magnetic oscillations at the resonance frequency of the atomic nuclei.
The performance of the probe coil can be evaluated according to its quality value (Q-value), which indicates its bandwidth relative to a resonant frequency of interest. Q is inversely proportional to the resistance of the coil. Thus, a high-Q coil has lower thermal noise and so, if tuned to the frequency of the sample's nuclei, can detect their magnetic oscillations with high sensitivity. Accordingly, other things being equal, a probe coil with a higher Q-value can produce higher-sensitivity measurements than a probe coil with a lower Q-value.
One way to improve the Q-value of an NMR probe coil is by forming it with a superconducting material. The superconducting material can enhance the sensitivity of the coil by responding to relatively small magnetic fields of the sample. Unfortunately, however, the superconducting material can also create unwanted magnetic fields on the sample due to persistent direct currents that flow through it. These unwanted magnetic fields can degrade the homogeneity of the static magnetic field applied to the sample, which can prevent the NMR probe from obtaining well-resolved and high-sensitivity measurements.